Acknowledge that SkIcoCodec can rewind and test it.

Since SkIcoCodec has an SkCodec for its encoded images, backed by
SkMemoryStreams, the SkMemoryStream can always rewind, and will be
rewound by the sub-codec if necessary (now that SkBmpCodec and
SkPngCodec support rewinding).

Depends on https://codereview.chromium.org/1057483003/ and
https://codereview.chromium.org/1048423003/ (DIFFERENT ISSUES).

BUG=skia:3257

Review URL: https://codereview.chromium.org/1054603002
1 file changed